Polysilane positive photoresist materials and methods for their use

ABSTRACT

New polysilane copolymers comprise recurring units of --Si(X)(Y)-- and Si(A)(B)--, Si(X)(Y) being different from Si(A)(B), 
     wherein 
     X and Y together have 1-13 carbon atoms, and X and Y each independently is hydrogen, alkyl, cycloalkyl, phenyl, alkylphenyl, or phenylalkyl, with the proviso that only one of X and Y contains a phenyl moiety, or together X and Y are an alkylene group forming a ring with the adjoining Si atom, 
     and wherein 
     A and B together have 3-13 carbon atoms, and A and B each independently is alkyl or cycloalkyl, with the proviso (a) that when one of A and B is ethyl, the other is not methyl or ethyl, and (b) that when one of A and B is n-propyl and the other is methyl, X and Y are not both methyl. 
     Corresponding homopolysilanes are also provided. 
     Upon ultraviolet irradiation, they photodepolymerize to form volatile products. As a result, they represent a new class of photoresists which enable direct formation of a positive image eliminating the heretofore required development step.

The U.S. Government has rights in this invention pursuant to ContractNo. DE-AC04-76DP00789 between the U.S. Department of Energy and AT&TTechnologies, Inc.

BACKGROUND OF THE INVENTION

This invention relates to new photoresist materials which can be used todirectly achieve a positive image of a desired pattern on a substrateand to methods for their us which eliminate the conventional developingstep.

Printed circuit boards, ubiquitous components of modern electronicequipment, are manufactured by the millions using photoresists.Typically, the photoresist is a thin layer of a photoreactive monomerwhich polymerizes on exposure to light, changing from a soluble to aninsoluble form. Like a photographic plate, the image is a negative onedeveloped by solvent removal of the unexposed areas. This developmentstep contributes a significant amount to the overall cost of printedcircuit production. Its elimination could, therefore, reduce costs by aconsiderable amount.

Heretofore, the polysilane class of polymers has not been applied inphotoresist technology. However, polysilanes are known. For example,West et al, J.A.C.S. 103, 7352 (1981) discloses (phenylmethylcodimethyl)silane as a solid film which crosslinks under UV irradiation. Ishikawaet al, J. Organometallic Chem., 42, 333 (1972) discloses the preparationof permethylpolysilanes which degrade to non-volatileoctamethyltrisilane and other polymeric materials upon exposure to UVirradiation in solution. Wesson et al, J. Poly. Sci., Polym. Chem. Ed.,7, 2833 (1979) discloses the preparation of polydimethylsilane. Wessonet al, op. cit., 19, 65 (1981) discloses block copolymers ofethylmethyl- and dimethyl-silane units. Wesson et al, op. cit., 18, 959(1980) discloses the preparation of copolymers of ethylmethylsilaneunits with dimethylsilane units as well as of methyl/propyl units withdimethyl units. In addition, U.S. Pat. Nos. 2,544,976, 4,052,430,2,612,511 and 4,276,424 disclose preparation of other silane-typephotoresists.

There remains a need to provide photoresists which are simpler andeasier to use and which have the properties desirable in thephotopatterning process used, e.g., in the microelectronics industry. Atthe same time, it is desirable to extend the range of uses of thepolysilanes into this area.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide newphotoresist materials which can be used directly to form a positiveimage of a desired pattern on the surface of a substrate.

It is another object of this invention to provide such positivephotoresist materials which are based on polysilanes.

It is also another object of this invention to provide such polysilanesand new uses therefor.

Upon further study of the specification and appended claims, furtherobjects and advantages of this invention will become apparent to thoseskilled in the art.

These objects have been attained by providing polysilanes comprisingrecurring units of

    --Si(X)(Y)--

wherein

X and Y together have 4-13 carbon atoms,

and

X and Y each independently is hydrogen, alkyl, cycloalkyl orphenylalkyl, or together X and Y are an alkylene group forming a ringwith the adjoining Si atom, with the proviso that only one of X and Ycontains a phenylalkyl group.

These objects have also been attained by providing polysilane copolymerscomprising recurring units of --Si(X)(Y)--and --Si(A)(B)--, Si(X)(Y)being different from Si(A)(B), wherein

X and Y together have 1-13 carbon atoms,

and

X and Y each independently is hydrogen, alkyl, cycloalkyl, phenyl,alkylphenyl, or phenylalkyl, with the proviso that only one of X and Ycontains a phenyl moiety, or together X and Y are an alkylene groupforming a ring with the adjoining Si atom,

and wherein

A and B together have 3-13 carbon atoms,

and

A and B each independently is alkyl or cycloalkyl, with the proviso (a)that when one of A and B is ethyl, the other is not methyl or ethyl, and(b) that when one of A and B is n-propyl and the other is methyl, X andY are not both methyl.

These objects have further been attained by providing a photofabricationmethod wherein X and Y each is methyl, and one of A and B is methyl andthe other is cyclohexyl, n-hexyl or n-dodecyl; or wherein one of X and Yis methyl and the other is n-propyl, and one of A and B is methyl andthe other is isopropyl.

These objects have been provided, in preferred aspects by providinghomopolysilanes wherein X and Y each is methyl, ethyl, phenethyl,isopropyl, n-propyl, t-butyl, n-hexyl, or n-dodecyl, or X and Y togetherform a pentamethylene group; by providing polysilane copolymers whereinX and Y each is methyl, ethyl, or propyl, or X and Y together formpentamethylene; and wherein one of A and B is n-propyl, isopropyl,t-butyl, cyclohexyl, n-hexyl, or n-dodecyl and the other is methyl orethyl; by providing polysilane copolymers wherein X and Y each ismethyl, and one of A and B is cyclohexyl, n-hexyl or n-dodecyl; orwherein one of X and Y is methyl and the other is n-propyl, and one of Aand B is methyl and the other is isopropyl; and by providing suchpolysilanes and a corresponding photopatterning method whereby a highresolution of 1-4 μm or higher is provided during the photoresistprocessing.

These objects have further been achieved by providing a correspondingprocess comprising coating a substrate, photopatterning it and removingthe coating from the substrate in a photoresist-type process; they havefurther been attained by providing compositions comprising the mentionedphotoresist polysilanes along with other conventional ingredients usedin photoresist processing, and also by providing the products of suchprocesses as well as the combination of substrates, typicallymicroelectronic substrates, and polysilane coatings.

DETAILED DISCUSSION

All of the polysilane polymers of this invention are useful for severalpurposes. For example, these polysilanes can be formulated withconventional, usually commercially available, crosslinking agents. Suchagents can be coated onto surfaces, articles, etc. to provide protectivelayers. Furthermore, the polysilanes are useful in processes involvingphotopatterning, e.g., in conventional photolithography and photoresistapplications. These include the patterning of decorative features on awide variety of substrates, low resolution patterning of preselecteddesigns on such substrates and high resolution patterning of highlycomplex patterns thereon, e.g., those required in the microelectronicsindustry. A major advantage of this invention for all such uses is thatthe desired pattern is produced in a positive image as will be explainedmore fully below. This eliminates the costly and time consumingdevelopment step required in the prior art photopatterning techniques.The high resolution photopatterning processes are preferred in thisinvention; consequently, the following discussion will be framedprimarily in terms appropriate for this use. However, this is notintended to limit the scope of this invention or to exclude the otheruses of the polysilanes.

In general, for high resolution applications, the polysilane will have ahigh photosensitivity under actinic radiation resulting inphotodepolymerization of the irradiated areas. The products of thephotodepolymerization step must be sufficiently volatile at thepolysilane temperature so that the products vaporize, thereby exposingthe underlying substrate. In addition, the polysilane must be formableinto a film and must have a low crystallinity in film form. Otherwise,there will be significant light scattering effects which will degraderesolution. Of course, the polysilane structure must be of a nature thatthe film does not crosslink under actinic radiation, preferablyultraviolet radiation. Various combinations of substituents on thesilicon-atom backbone of a polysilane molecule are effective to achievethese properties to varying degrees. Suitable polymers for any of theforegoing uses and having a desired combination of properties can beconventionally prepared, perhaps with a few routine preliminaryexperiments using the guidelines discussed herein.

Polysilane homopolymers will have two substituents on each silicon atom,denoted as X and Y. Preferably, these substituents together will have4-13 carbon atoms. Combinations of X and Y substituents having fewerthan 4 carbon atoms in total tend to be highly crystalline and are notsatisfactory for use in high resolution processes. However, these areincluded within the scope of this invention in methods which do notdemand high resolution. For example, they can be successfully used invery low resolution processes such as those wherein decorative effectsare to be achieved on substrates. Light scattering due to a highcrystallinity of a polymer can even be of advantage in suchapplications, e.g., in view of the interesting effects which can beproduced thereby. Polysilanes wherein X and Y in combination have atotal number of carbon atoms greater than 13 are also within the scopeof this invention. However, such homopolymers will depolymerize to formrelatively heavy fragments requiring relatively high temperatures forvolatilization. Applications wherein such high temperatures can be used,however, are within the scope of this invention; consequently, thecorresponding polysilanes which decompose into fragments whichvolatilize only at such high temperatures, e.g., on the order of 200° C.or higher, are contemplated as equivalents within the scope of thisinvention. Similarly, also within the scope of this invention are othersubstituents X and Y which are not based upon hydrocarbon moieties,especially those bonded to Si atoms via C-atoms.

Typically, the substituents X and Y will be selected independently amonghydrogen, alkyl, cycloalkyl or phenylalkyl groups. Together, X and Y canalso form alkylene groups linking with the connecting silicon atom toform a ring. Polysilanes wherein neither X nor Y is hydrogen arepreferred. Hydrogen atoms on the polymer chain cause a high tendencytoward reaction with oxygen making the polymers very difficult tohandle. However, where such constraints are not a problem, hydrogensubstituents are employable.

Suitable alkyl groups have 1-12 carbon atoms, e.g., include methyl,ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, t-butyl,n-pentyl and its isomers, n-hexyl and its isomers, n-heptyl and itsisomers, n-octyl and its isomers, n-nonyl and its isomers, n-decyl andits isomers, n-undecyl and its isomers, and n-dodecyl and its isomers.Similarly, suitable cycloalkyl groups have 3-12 carbon atoms and includeall of the various possibilities derived from the corresponding alkylgroups, e.g., those mentioned above, including those cycloalkyl groupshaving alkyl substituents. Preferably, the cycloalkyl group iscyclopentyl, cyclohexyl, cycloheptyl, etc., most preferably cyclohexyl.Suitable phenylalkyl groups are those based on the mentioned alkylgroups, e.g., the C₁₋₆ -alkyl groups. The alkylene groups formed by Xand Y together also encompass all of the various possibilities derivedfrom the corresponding alkyl groups mentioned above, i.e., are of 2-12carbon atoms, including those having alkyl substituents. As can be seen,the Si atoms can be bonded to primary, secondary or tertiary C-atoms.

The foregoing discussion of suitable groups is not meant to be preciselylimiting on the number of carbon atoms possible. As discussed above,under appropriate circumstances and for suitable uses, moieties having agreater number of carbon atoms will be suitable and, consequently, arecontemplated as equivalents within the scope of this invention.

The substituents X and Y are selected to provide the proper balance, atthe polysilane temperature during irradiation, between photosensitivityto actinic radiation and volatility of the fragments which result fromthe photodepolymerization. That is, the substituents X and Y must be ofsufficient bulkiness to provide significant steric hindrance which, itis believed, contributes toward destabilization of the polysilane.However, they must also be sufficiently lightweight to producesufficiently small fragments to be volatile at the instantaneous localtemperature of the polymer during the irradiation period. As mentioned,selection of appropriate substituents for the intended end use can beachieved easily, perhaps with a few routine preliminary experiments, inaccordance with this discussion.

For example, cyclohexyl-methyl homopolysilane has good photosensitivitytoward depolymerization due to the presence of the cyclohexyl group. Itundergoes only low ablation at room temperature but at elevatedtemperatures has good ablation properties. Homodi-n-hexyl polysilaneundergoes photodecomposition but does not ablate well at roomtemperature; however, at elevated temperatures its ablation isincreased, whereby good photopatterning can be achieved. The polymerdoes have considerable crystallinity which limits resolution. Theproperties of homomethyl-n-dodecyl polysilane are very similar to thosedescribed for the di-n-hexyl homopolymer. Homomethylphenethyl polisilanedoes undergo photodepolymerization and will ablate at temperatures aboveroom temperature.

Homopolymers based on X and Y moieties which are methyl and/or ethyl ineach case tend to be highly crystalline, again limiting the achievableresolution. In addition since these substituents are relatively small,photosensitivity is not high. However, each of these polymers can beemployed in one of the uses which places low demands on resolution,volatility, speed, etc. Some of these homopolymers which have been madeinclude those wherein X and Y are both methyl (highly insoluble) andwherein one is methyl and the other is ethyl. Crystallinity is alsoobserved for homopolymer films wherein one of X and Y is methyl and theother is isopropyl (much less crystalline than the dimethylhomopolymer). Homopolymers containing Si-aryl moieties tend to be toophotoinsensitive to provide sufficient depolymerization under theinfluence of ultraviolet radiation, presumably because of regularitiesin the alignment of the pendant phenyl groups, i.e., due to low sterichindrance effects. For instance, homomethylphenylpolysilane is veryphotoinsensitive in the solid state throughout the ultraviolet region.

The foregoing discussion of typical homopolymers demonstrates the natureof size effects on both photosensitivity and volatility. Furtherguidance in selecting an appropriate homo- or co-polysilane for a givenuse can be obtained from routine studies of a polysilane'sphotosensitivity in solution. However, there is no direct correlationwith the photosensitivity of a corresponding film.

In various preferred aspects, the homopolymers of this invention have Xand Y moieties which together have at least 5, 6, 7 or 8 carbon atomsand also which have a maximum of 12, 11, 10, or 9 carbon atoms. Ingeneral, it is preferred that each of X and Y be selected among methyl,ethyl, phenethyl, isopropyl, n-propyl, t-butyl, n-hexyl and n-dodecyl.

It is preferred that the polysilane used in this invention be acopolymer of recurring units (--Si(X)(Y)--Si(A)(B)--)_(n). In this way,one set of units can be selected to appropriately tailor thephotosensitivity of the polysilane towards photodepolymerization and theother can be used to appropriately tailor the volatility of thefragments which result. That is, one unit will be selected to besufficiently bulky to provide sufficient steric hindrance that thepolysilane photodepolymerizes, while the other will be selected to besufficiently lightweight that the resultant fragments volatilize at thetemperature of the polysilane during actinic irradiation. Moreover, thecopolymers are preferred because they have a lower tendency towardcrystallinity.

In general, the entire discussion above regarding the structure of thehomopolymers applies to the copolymers unless indicated otherwise.

In the following description of suitable polysilane copolymers, themoieties X and Y are used to define the volatile component. Preferably,this component is one in which both X and Y are methyl. The suitabilityof employing dimethyl units will, of course, depend upon the nature ofthe A/B units used. The smaller the size of the substituents in thevolatile unit, the greater is the tendency toward crystallization. Thistendency will be exacerbated when the A/B moieties are also relativelysmall in size. However, in general, the volatile units can have X and Ymoieties of a total number of carbon atoms in the range of 1-13. Evenlarger moieties can be employed under appropriate conditions asindicated above, primarily when high polysilane temperatures areachieved during irradiation and/or when the A/B unit is of relativelysmall size. Consequently, X/Y moieties having a total number of carbonatoms greater than 13 are contemplated equivalents within the scope ofthis invention.

As mentioned above, it is preferred that neither X nor Y be hydrogen.Suitable alkyl and cycloalkyl moieties include those mentioned above.These include the alkyl portions of the alkylphenyl and phenylalkylmoieties as long as the carbon atom limitations are observed. The samealkylene groups formed by X and Y together in the homopolymers areemployable in the copolymers.

The unit denoted as having A and B substituents is the photosensitivitycontributor. Again, the stated total carbon atom range of 3-13 isgenerally appropriate. However, under some circumstances moieties havingfewer than 3 or greater than 13 carbon atoms could be successfullyemployed; consequently, they are contemplated as equivalents within thescope of this invention. Suitable alkyl and cycloalkyl groups are thosedescribed above. Typically, the photosensitivity contributor will haveat least one relatively bulky substituent such as isopropyl, t-butyl,n-hexyl, n-dodecyl, any of the branched and unbranched alkyl groupsintermediate in C-number between the latter two alkyl groups,cyclohexyl, cyclopentyl, cycloheptyl, etc.

For example, the preferred copolysilane of this invention is one whereinboth X and Y are methyl and wherein one of A and B is methyl and theother is cyclohexyl. It has a high photosensitivity tophotodepolymerization and produces volatile fragments as discussed morein detail below. This polymer has a ratio of X/Y units to A/B units of1/1. When the ratio of dimethyl units to methylcyclohexyl units is about1/4, the resulting copolymer has good photosensitivity, but, due to theincreased fragment weight caused by the increased relative content ofcyclohexyl groups, the copolymer does not ablate well at roomtemperature. Nevertheless, this copolysilane is useful within the scopeof this invention, especially when higher polysilane temperatures areinvolved during the irradiation period. The complementary copolymerwherein the ratio of dimethyl units to cyclohexylmethyl units is 4/1 haslower photosensitivity but produces depolymerization products which arevolatile at lower temperatures. In addition, because of the increasedcontent of methyl groups, the polymer has an increased crystallinitywhich lowers the achievable resolution. Nevertheless, the polymer isvaluable for many of the less demanding uses discussed herein.

The copolymer of dimethyl units and isopropylmethyl units ort-butylmethyl units is highly crystalline. However, these polymers canbe used within the broad scope of this invention in view of theirinherent photosensitivity and the volatility of the depolymerizationfragments. Also highly crystalline are copolymers of ethyl/methyl unitswith dimethyl units and of methyl/n-propyl units with dimethyl units.These copolymers similarly can be employed within the broad scope of theuses of this invention. The copolymer of phenylmethyl units withdimethyl crosslinks under the influence of ultraviolet radiation makingits use within the scope of this invention problematic.

As noted above, the range of suitable carbon atom content for the X/Ymoieties of this invention can preferably be defined as a minimum of2,3,4,5, or 6 or as having a maximum of 12,11,10,9,8 or 7. Similarly, Aand B together can have a total preferred carbon atom content of 4,5,6or 7 or a preferred maximum carbon atom content of 12,11,10,9 or 8.

The molar ratio of the X/Y units to the A/B units is not critical. Ingeneral, this ratio is limited at the upper end (increasingvolatility-unit content) by the tendency toward crystallizationcontributed by the relatively smaller size of the X and Y moieties.Simlarly, as this ratio gets larger, the photosensitizing effectcontributed by the A/B groups may become too small. At the lower end(increasing A/B content), the ratio is limited by a tendency of thedepolymerization fragments to remain nonvolatile even at elevatedtemperatures. However, in most cases, a very broad range of suitableratios will be useful and can be routinely selected for each polymer.

Similarly, the molecular weight of the polysilanes is also uncritical.Typically, the number of monomer units in a polymer varies from 3 to20,000 or 50,000. All chain lengths will be photosensitive as long assufficiently short wavelength actinic radiation is utilized, e.g., about250 nm or shorter. Molecular weights up to 2,000,000 or more can beinvolved with no problems. In general, as the molecular weightincreases, the maximum wavelength effective to causephotodepolymerization will also increase. Consequently, as thephotodepolymerization process ensues, the maximum effective wavelengthwill decrease to shorter values, i.e., the ultraviolet energy necessaryto continue the photodepolymerization process will increase. For thisreason, actinic radiation well below the maximum effective wavelength atthe beginning of a process will be employed. As mentioned, wavelengthsless than about 250 nm will usually be effective for this purpose.Nevertheless, if desired, wavelengths throughout the ultraviolet rangecan be utilized in dependence upon the particular polymer employed inaccordance with the foregoing.

Although U.V. radiation is preferred, actinic wavelengths in the visiblerange are also possible, as are higher energy radiations such as X-rayor gamma ray and charged particle beams, e.g., electron beams. A verywide variety of actinic radiation sources can be used, spanning a verybroad range of low intensity devices to very high power, e.g., laserdevices. The preferred mode uses laser irradiation, e.g., from a pulsedexcimer laser, typically of an intensity of 1×10⁻⁶ to 1 J/cm², higher orlower values being employable depending on whether local heating isdesired and on other factors discussed herein or obvious to skilledworkers. Of course, irradiation with high power sources has theadvantage that polysilanes of relatively lower photochemical sensitivitycan be employed. These are more easily handled than are the morereactive polysilanes.

In general, the polymers will be photodepolymerized at about roomtemperature. That is, actinic radiation intensities will be selected tocause photodepolymerization quickly without significantly thermallyaffecting the remaining polysilane. However, where desirable, e.g., inorder to increase the volatility of photodepolymerization products,increased polysilane temperatures can be used during actinicirradiation. The polysilane temperature can be increased by simple,conventional heating of the substrate and/or the polysilane coatingitself, e.g., by inductive, conductive or radiative means. It can alsobe increased by employing high energy, pulsed or continuous wave lasersor other high intensity sources which will raise the instantaneous localtemperature of the polysilane molecules undergoingphotodepolymerization.

The volatile products of the depolymerization reactions areenvironmentally benign. In vacuo, the fragments are simple low molecularweight silylenes such as dimethyl silylene and higher homologues, independence upon the precise structure of the recurring units, or aresimple subunits of the polymeric chain, i.e., typically monomers,dimers, trimers, tetramers, etc. In air, the products are thecorresponding oxidized fragments, e.g., the corresponding low molecularweight siloxanes. The siloxanes are physiologically innocuous and poseessentially no risk to personnel. Similarly, they are non-corrosive andpose essentially no risk to equipment as long as routine conventionalsafeguards are employed, e.g., adequate ventilation is provided. Theseare very important properties for the commercial utilizability of thepolysilanes, e.g., in photofabrication processes.

Without intending to limit the scope of this invention in any way, it isbelieved that photodepolymerization of the polysilanes used in thisinvention occurs via chain scission at the silicon atoms having the moststerically hindered substituents. UV absorption occurs via electronictransitions in the silicon atoms. The resultant high energy excitedstate is unstable and results in depolymerization producing thefragments discussed above. The resultant depolymerized area (substrate)is essentially completely flat, there being essentially no observabletraces of polymer fragments.

It has been discovered that this depolymerization process is a highlyefficient one, e.g., a quantum yield of about 6 has been measured forthe depolymerization of (cyclohexylmethyl Si/dimethyl Si)_(n). Thiscompares very favorably with quantum yields of 0.1 to 0.4 for commercialphotoresists. The high yield for this invention suggests a free radicalmechanism in the unzipping of the chain, most probably, it is theorized,via the intermediacy of diradicals. Furthermore, the fluorescencequantum yield for the same polymer is about 0.6, indicating that 40% orless of the impinging photons actually result in an effectivedepolymerization absorption. On this basis alone, quantum yieldsimproved by a factor of about 2 will be achieved by using polymershaving decreased fluorescence yields, a parameter which can be readilymeasured for any candidate polymer. A further increase in quantum yieldcan be achieved by increasing the temperature of the polysilane sincediradical processes are generally observed to be strongly temperatureactivated. Overall, it can be seen that the photo-fabrication processesof this invention will proceed with heretofor unachievable efficiency.

The properties of the polysilanes used in this invention are excellentfor the wide variety of photo-fabrication and other uses mentionedherein. As discussed, they have widely varying crytallinities independence upon the structure of the monomeric units incorporated. Highcrystallinity will be tolerable or even desired when attractivedecorative effects are to be patterned in a substrate. For higherresolution imaging, of course, a very low or essentially zerocrystallinity will be desired, e.g., typically causing less than about20% scattering of incident visible radiation and preferably much less.

The polysilanes of this invention also have high stability properties,e.g., mechanically, chemically and thermally. The mechanical stabilityof the polymers is retained when the polymers are coated onto a widevariety of substrates, including quartz, glass, metals, e.g., siliconand other semiconductor substrates, ceramics, polymers, etc. Thepolysilanes adhere well to all such surfaces, without any specialsurface pretreatments. Mechanical stability as well as chemicalstability is also retained after exposure to the common etchantsemployed in photofabrication processes, e.g., ferric chloride, stannouschloride, etc. The polymers have good thermal stability. For example,differential scanning calorimetry has revealed thecyclohexylmethyl/dimethyl copolymer to be thermally stable to 390° C.Thermogravimetric methods have shown no weight loss for this polymer upto about 220° C., and some weight loss (about 10%) only at about 325° C.probably due to the presence of lower molecular weight chains(oligomers). Consequently, this polymer will be employable in hightemperature processing. Similar results are observed for the otherpolysilanes, precise results being routinely determinable in each case.

Another important requirement for certain photofabrication processes,e.g., in the semiconductor industry, is a low content of trace metals,e.g., sodium cations derived from the preparative methods discussedbelow. Routine purification procedures and/or modifications of thepreparative chemistry can readily achieve the necessarily low metalcontamination concentrations, e.g., less than about 10 ppm or lower.Many other properties of the polysilane films of this invention are alsoexcellent for the intended purposes.

Because of the wide variety of structural combinations available in thepolysilanes of this invention, many of these properties can be finetuned for an intended end use. This is especially the case for thepolysilane copolymers, which provide at least two basic monomeric unitsfor structural variation. The copolymers, of course, can comprise morethan two basic units, i.e., the copolymer can comprise not only onevolatility- contributing unit and one photosensitivity-contributing unitbut also 3, 4 or more different recurring monomeric units. This providesa high tailorability of properties.

As mentioned above, this invention is also directed to the use of thepolysilanes. All these polymers can be used for the conventional purposeof providing protective coatings on a wide variety of substrates. Suchcoatings can be conventionally prepared by incorporating a conventionalcrosslinking agent in effective amounts into the polysilane coating.Upon UV irradiation, a crosslinked protective coating will result. Theyalso can be used conventionally as impregnating agents for strengtheningceramics, precursors for beta-SiC fibers, and as dopable semiconductors.See, e.g., J. Am. Chem. Soc., 103, 7352 (1981); J. Am. Ceram. Soc., 61,504 (1978); and Chem. Lett., 551 (1976).

However, primarily, this invention is directed to the use of thepolysilanes in photofabrication, e.g., in the application of positiveimage patterns onto substrate surfaces, that is as a new class of uniquephotoresists. The uniqueness is derived from the fact that upon exposureto a pattern of actinic radiation, the polysilanes depolymerize todirectly expose the underlying surface in the same pattern. Thiseliminates the development step heretofor necessary in conventionalphotoresist technology.

In one use, the photoresists of this invention are applied to asubstrate and then irradiated with actinic radiation to produce adecorative pattern. In such applications, crystallinity of thepolysilane is often not a problem and can even be an advantage sinceunique decorative patterns can be produced by the scattering of light bythe crystal centers. More preferably, the photoresists of this inventionare used in low resolution imaging where integrity of an irradiatedpattern is required but line widths are relatively large, e.g., on theorder of many tens or hundreds of micrometers. In this application,crystallinity is also often not a problem.

Most preferably, this invention is directed to high resolution imagingusing the photoresists of this invention. Here, resolution of a fewmicrons is necessary (e.g., about 2-10 μm; VLSI applications) or evenlower (ULSI applications).

For all the photofabrication techniques of this invention, the generalmethod is to coat the desired substrate with a photoresist of thisinvention. The image of the desired pattern is then focused on thecoating whereby those portions of the underlying coating on which theactinic radiation impinges are depolymerized, exposing the underlyingsurface. The exposed surface is then treated as desired, whereupon theremaining photoresist is removed by flooding the entire surface withactinic radiation causing the entire photoresist to ablate. As can beseen, this general method not only eliminates the heretofor necessarydevelopment step, but also replaces the previous chemical stripping ofthe remaining photoresist by the much simpler photolytic method. Thestripping step can also be carried out using solvents if desired, e.g.,those employable in the polymerization of the polysilanes (see below) orothers in which the polymers are soluble (see e.g., Example 1 below).

The polymers of this invention can be coated onto any substrate usingconventional means such as pouring, dipping, brushing, spreading,spraying, spin casting, etc. The polymers can be applied neat ordissolved in a compatible inert solvent such as THF, chlorinatedhydrocarbons, toluene, xylene, hexane, etc. The coatings can be dried atroom temperature over a period of hours, e.g., 3-4 hours, or can beheated or sprayed to set at shorter times. Storage of the polymers isnot a problem unless long times are involved whereupon conventionalmeans to protect them from actinic radiation can be taken, e.g., theycan be stored in foil-covered or otherwise light blocking vials.

The polysilanes can be applied in any appropriate thickness, e.g., fromabout 1 μm to about 1 mil. The thickness will be chosen in accordancewith the desired resolution, smaller thicknesses generally beingemployed when higher resolutions are desired. For example, for aresolution of about 1 μm, a thickness of about 1 μm will suffice.Thickness is controlled conventionally, e.g., by varying concentration,the number of layers are deposited, etc.

Unlike many photoresists, the photoresists of this invention are usuallyapplied without the copresence of photosensitizers. However, of course,if desired, e.g., if it is desired to use a particular light source towhich the polysilanes per se are not sensitive, conventionalphotosensitizers can be employed in conventional amounts.

Irradiation times will be very short, e.g, on the order of seconds ormuch shorter in dependence upon the intensity of the actinic radiationsource and layer thickness. As mentioned, the nature of the source isnot critical, low power (e.g., 10⁻⁶ J/cm² or lower) or high power (1 ormore J/cm²) devices being employable such as continuous wave or pulselasers or lamps. The optics associated with the actinic radiation arefully conventional. Preferably, ultraviolet radiation is used, generallylower than 330 nm and preferably lower than 254 nm, e.g., using a KrFexcimer laser at 248 nm.

A particularly preferred application of the photofabrication techniquesof this invention is in the electronics industry in making printedcircuit boards, computer chips, etc. However, the polysilanes may alsobe used for non silver-based photography since the quantum yields are sohigh. For example, in one method, a crosslinking agent will be includedin the polysilane coating. Upon exposure to patterned actinic radiation,a photographic positive image will be preliminarily set in the coating.Thereafter, by appropriate heating of the remaining polymer containingthe conventional cross-linking agent, the positive image will be finallyset therein. In a related application, the polysilane films of thisinvention can be used as interlayer dielectrics, again by including inthe layers conventional cross-linking agents. Upon exposure to patternedactinic radiation, the resultant positive image can be finally set inthe dielectric layer by conventional heating of the remaining polymer.This eliminates the now required additional steps required to achievesuch a positive image in a dielectric layer in state-of-the-artmicroelectronic devices.

The polysilanes of this invention are prepared by processes which areessentially conventional. These are generally described in many of thereferences cited above. In essence, the corresponding monomerichalosilanes (preferably the dichlorosilanes) are polymerized in thepresence of an alkali metal catalyst, preferably sodium, in an inertsolvent such as toluene, preferably at elevated temperatures, e.g.,90°-100° C. under reflux. The solvent has a minor effect on theresultant polymers via its influence on the nature of the end groups ofthe chains. It is desirable that the sodium be added to the reaction ina uniform manner. This results in a more homogeneous molecular weightdistribution. The precise rate of sodium addition is not critical.Molecular weight can also be controlled by addition of small amounts ofmonohalosilanes as chain terminators. All of the polysilanes can beprepared analogously to the methods described in the following examples.Also see the following which report some of the preparations which are apart of this invention; J. Poly. Sci., Polym. Lett. Ed., 21, 819 (1983);ibid, 823; and J. Poly. Sci., Polym. Chem. Ed., 22, 159 (1984); ibid,225. As disclosed in the following examples and in all of the referencesmentioned herein, conventional fractionation and other treatments areused to further purify the initially obtained polysilanes, e.g., toeliminate low molecular weight oligomers or cyclic materials and, aswell, residual alkali metal cations.

The starting material silanes, preferably the correspondingdichlorosilanes are mostly commercially available either as stock itemsor as specialty chemicals. All can be routinely prepared by conventionalGrignard addition to chlorosilanes. Silanes other than halosilanes canalso be used and are commercially available or readily preparable fromknown starting materials.

In certain less preferred aspects, this invention can exclude thefollowing subclasses of polysilanes per se but not their use inaccordance with the preferred methods of this invention. In onesubclass, there are excluded homopolymers wherein X/Y aremethyl/p-tolyl; in another, copolymers wherein X/Y arebeta-phenethyl/methyl and A/B are cyclohexyl/methyl or wherein X/Y arep-tolyl/methyl and A/B are cyclohexyl/methyl or wherein X/Y togetherform pentamethylene and A/B are cyclohexyl/methyl. In another, there areexcluded homopolymers wherein X/Y are beta-phenethyl/methyl, ormethyl/cyclohexyl, or methyl/phenyl, or methyl/n-hexyl ormethyl/n-dodecyl or wherein X/Y together form pentamethylene. In yetanother, there are excluded copolymers wherein X/Y are methyl/methyl andA/B are methyl/cyclohexyl, or wherein X/Y are methyl/methyl and A/B aremethyl/n-hexyl.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever. In the followingexamples, all temperatures are set forth uncorrected in degrees Celsius;unless otherwise indicated, all parts and percentages are by weight.

EXAMPLE 1 Synthesis, Purification, Characterization and Properties of(Cyclohexyl Methyl Si/Dimethyl Si)_(n)

Purified cyclohexylmethyldichlorosilane (48.9 g, 0.248 mole) andpurified dimethyldichlorosilane (32.0 g, 0.248 mole) are added to 500 mlof dry toluene in an oven dried, N₂ -purged, 3-necked flask equippedwith reflux condenser, N₂ inlet, stirring bar, and pressure equalizingdropping funnel. The toluene solution of monomers is heated to refluxand a mineral oil dispersion of sodium metal (62.74 g of 40% by weightdispersion, 1.091 g-atom) is added cautiously from the dropping funnelat the most rapid and constant rate permitted by the highly exothermicreaction. Throughout the course of the synthesis and purification of thepolymer, it is protected from light to prevent prematurephotodegradation. Reflux of the dark blue reaction mixture is continuedfor 4 hours after sodium addition is completed. Neat methanol is thenadded cautiously to the cooled reaction mixture until H₂ evolution fromthe destruction of excess Na stops. A volume of saturated aqueous sodiumbicarbonate equal to the volume of the reaction mixture is added in asingle portion and the resulting two-phase mixture stirred vigorouslyuntil the blue color is completely dissipated. After separation of thelayers, the cloudy organic phase is filtered through filter aid toremove small amounts of insoluble polymer. Removal of the toluenesolvent from the filtrate at reduced pressure affords a yellow-brownviscous oil. Crude high molecular weight (cyclohexyl methyl Si/dimethylSi)_(n) is precipitated from this oil by addition of ten volumes ofethyl acetate. Filtration gives a slightly tacky polymer which ispurified by one precipitation from toluene with ethyl acetate and twoprecipitations from tetrahydrofuran with methanol. This procedureprovides 5.56 g of pure white, flocculent (cyclohexyl methyl Si/dimethylSi)_(n). Gel permeation chromatography analysis of this material showsit to possess a trimodal molecular weight distribution (modes at 5000,40000 and 300000 daltons). Spectral data consistent with the copolymerstructure were obtained as follows: UV (nm): λmax 304 (ε6400), λmax223(ε3100); ¹ HNMR (relative to tetramethylsilane reference): δ1.3 (br.s, Si-CH₃), 1.8 and 2.3 (br. m., Si-cyclohexyl); IR (cast film) (cm⁻¹):2940, 2880, 1458, 1250, 833, 750, 728. The copolymer is soluble inchlorinated hydrocarbons, aromatic hydrocarbons, and tetrahydrofuran,moderately soluble in alkanes and cycloalkanes, and slightly soluble inether, acetone, ethyl acetate, and isopropanol. It is essentiallyinsoluble in water, methanol, and other highly polar organic solvents.Thermogravimetric analysis shows the material to suffer essentially noweight loss to 220° C., 10% at 325° C., and 50% loss at 380° C.Differential scanning calorimetry shows no detectable transitions (Tg orTm) from -140° C. to the strong decomposition exotherm at about 390° C.Atomic absorption analysis indicated the above sample to contain 80 ppmNa by weight.

Elemental Analysis: C=53.78%; H=10.84%; Si=31.15%;

Theoretical Si content assuming a 1/1 molar ratio of cyclohexylmethylunits to dimethyl units is 35.31%.

All other polysilanes are preparable analogously.

EXAMPLE 2

Casting of Films of (Cyclohexyl Methyl Si/Dimethyl Si)_(n)

Depending on the thickness desired, a solution of the above copolymer inthe 0.1-1% concentration (w/w) range in toluene is prepared andparticulates removed by passing the solution through a 0.2 μm membranefilter. The solution is then applied to the substrate (e.g., glass,quartz, silicon, aluminum, copper) and allowed to evaporate slowly whileprotected from light. Alternatively, films can be prepared by spincasting. Films of submicron thicknesses up to 1 mil or more can beprepared by these methods and possess excellent adherence to all of theabove substrates.

EXAMPLE 3 Patterning of (Cyclohexyl Methyl Si/Dimethyl Si)_(n) Films

A 1 μm thick film of the above polymer on metallic silicon is patternedwith pulsed ultraviolet light from a KrF excimer laser (2480 Å max, 10 Åfull width at half height) and energy levels of 10⁻⁶ J/cm², using aquartz mask. Approximately a thousand 10 nanosecond pulses aresufficient to completely ablate a 1 μm film in the exposed area. Thepolymer films have also been patterned successfully with a low intensityhand-held low pressure Hg lamp (2537 Å) in less than 60 seconds.

EXAMPLE 4

Analogously to Example 1, homopolysilanes and copolysilanes having alkylor aryl portions as described above but with substituents bonded theretohave been prepared, e.g., homopolysilanes of methyl/p-methoxyphenyl andothers. Suitable substituents on the alkyl, cycloalkyl, alkylene orphenyl moieties include C₁₋₆ -alkoxy, mono- or di(CC₁₋₃ -alkyl)-amino,C₂₋₆ -alkanoyl or the corresponding C₂₋₃ -ketals, C₁₋₆ -alkyl asmentioned above, NH₂, and OH, e.g., methoxy, ethoxy, etc., methylamino,ethylamino, etc., diethylamino, dimethylamino, etc., acetyl, propanoyl,etc., and the corresponding ethylene or propylene ketals, methyl, ethyl,etc., amino, hydroxy, etc. Generally, suitable alkyl portions of thesesubstituents are those mentioned above in conjunction with the X, Y, Aand B moieties. Also prepared were polymers having naphthyl groupsattached to the Si-backbone, e.g., β-naphthyl/methyl homopolymer or 100(phenyl/methyl)/1 β-naphthyl/methyl copolymer. α-naphthyl groups canalso be used. The naphthyl groups can also be substituted as described.p-phenyl substitution is preferred. Usually, there is only onesubstituent per pendant group, but additional substitution iscontemplated as an equivalent.

The analogous preparation of these substituted or naphthyl-containingpolymers is varied simply by employing the corresponding substituted ornaphthyl-containing starting material halosilanes which are allconventionally preparable and/or commercially available. In addition,where necessary or advisable, as will be readily recognized by skilledworkers, groups such as alkanoyl can be conventionally protected duringthe polymerization, preferably with readily cleavable ketone protectivegroups. Amino and hydroxy groups can also be protected but this isusually not necessary.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

What is claimed is:
 1. A polysilane which is substantially free of lowmolecular weight oligomers or cyclic materials and whcih comprisesrecurring units of

    --Si(X)(Y)--

wherein X and Y together have 4-13 carbon atoms, and X and Y eachindepedently is hydrogen, alkyl, cycloalkyl, substituted phenyl, 1- or2-naphthyl, or phenylalkyl, or together X and Y are an alkylene groupforming a ring with the adjoining Si atom, wherein X and Y groups whichare not H can optionally be substituted by one of C₁₋₆ -alkoxy, mono- ordi-(C₁₋₃ -alkyl)amino, C₂₋₆ -alkanoyl or the corresponding C₂₋₃ -ketalthereof, C₁₋₆ -alkyl, --NH₂ or --OH; with the proviso that only one of Xand Y can be substituted phenyl, and wherein, as a result of theselection of X and Y, said polysilane has sufficient photosensitivity todepolymerize upon exposure to actinic radiation, forming products whichvolatilize.
 2. A polysilane of claim 1 which is substantiallynon-crystalline and wherein neither X nor Y is H.
 3. A polysilane ofclaim 1 wherein one of X or Y is cyclohexyl and the other is methyl. 4.A polysilane of claim 1 wherein X and Y each is methyl, ethyl,phenethyl, isopropyl, n-propyl, t-butyl, n-hexyl, or dodecyl, or X and Ytogether form a pentamethylene group.
 5. A polysilane of claim 2 whichis substantially not cross-linked upon exposure to actinic UV radiation.6. A polysilane copolymer comprising recurring units of --Si(X)(Y)--,and --Si(A)(B)--, Si(X)(Y) being different from Si(A)(B),wherein X and Ytogether have 1-13 carbon atoms, and X and Y each independently ishydrogen, alkyl, cycloalkyl, phenyl, alkylphenyl, or phenylalkyl, withthe proviso that only one of X and Y contains a phenyl moiety, ortogether X and Y are an alkylene group forming a ring with the adjoiningSi atom,and wherein A and B together have 3-13 carbon atoms, and A and Beach independently is alkyl, 1- or 2-naphthyhl, or cycloalkyl, whereinX, Y, A and B groups which are not H can optionally be substituted byone of C₁₋₆ -alkoxy, mono- or di-(C₁₋₃ -alkyl)amino, C₂₋₆ -alkanoyl orthe corresponding C₂₋₃ -ketal thereof, C₁₋₆ -alkyl, --NH₂ or --OH; withthe proviso (a) that when one of A and B is ethyl, the other is notmethyl or ethyl, and (b) that when one of A and B is n-propyl and theother is methyl, X and Y are not both methyl.
 7. A polysilane of claim 6which is substantially non-crystalline and wherein neither X or Y is Hor alkylphenyl.
 8. A polysilane of claim 7 wherein the ratio of--Si(X)(Y)-- to --Si(A)(B)-- is about 1 to
 1. 9. A polysilane of claim 6wherein X and Y are both methyl and wherein one of A and B is methyl andthe other is cyclohexyl.
 10. A polysilane of claim 6 wherein one of Xand Y is methyl and the other is n-propyl, and one of A and B is methyland the other is isopropyl.
 11. A polysilane of claim 6 wherein X and Yeach is methyl, ethyl, or propyl, or X and Y together formpentamethylene; and wherein one of A and B is n-propyl, isopropyl,t-butyl, cyclohexyl, n-hexyl or n-dodecyl and the other is methyl orethyl.
 12. A polysilane of claim 6 wherein X and Y each is methyl, andone of A and B is cyclohexyl, n-hexyl or n-dodecyl; or wherein one of Xand Y is methyl and the other is n-propyl, and one of A and B is methyland the other is isopropyl.
 13. A polysilane of claim 6 comprising atleast three different recurring units obtained from --Si(X)(Y)-- and--Si(A)(B)-- and including at least one of each said units.
 14. Apolysilane of claim 6, which is substantially free of low molecularweight oligomers or cyclic materials and which comprises said recurringunits, and wherein, as a result of the selection of A, B, X and Y, saidpolysilane has sufficient photosensitivity to depolymerize upon exposureto actinic radiation, forming products which volatilize.
 15. Apolysilane which is substantially free of low molecular weight oligomersor cyclic materials and which comprises recurring units of

    --Si(X)(Y)--

wherein X and Y together have 5-13 carbon atoms, and X and Y eachindependently is hydrogen, alkyl, cycloalkyl, substituted phenyl, 1- or2-naphthyl, or phenylalkyl, or together X and Y are an alkylene groupforming a ring with the adjoining Si atom, wherein X and Y groups whichare not H can optionally be substituted by one of C₁₋₆ -alkoxy, mono- ordi-(C₁₋₃ -alkyl)amino, C₂₋₆ -alkanoyl or the corresponding C₂₋₃ -ketalthereof, C₁₋₆ -alkyl, --NH₂ or --OH; with the proviso that only one of Xand Y can be substituted phenyl, and wherein, as a result of theselection of X and Y, said polysilane has sufficient photosensitivity todepolymerize upon exposure to actinic radiation, forming products whichvolatilize.
 16. A polysilane which is substantially free of lowmolecular weight oligomers or cyclic materials and which comprisesrecurring units of

    --Si(X)(Y)--

wherein X and Y together have 4-13 carbon atoms, and X and Y eachindependently is hydrogen, alkyl, cycloalkyl, 1- or 2-naphthyl, orphenylalkyl, or together X and Y are an alkylene group forming a ringwith the adjoining Si atom, wherein X and Y groups which are not H canoptionally be substituted by one of C₁₋₆ -alkoxy, mono- or di-(C₁₋₃-alkyl)amino, C₂₋₆ -alkanoyl or the corresponding C₂₋₃ -ketal thereof,C₁₋₆ -alkyl, --NH₂ or --OH; and wherein, as a result of the selection ofX and Y, said polysilane has sufficient photosensitivity to depolymerizeupon exposure to actinic radiation, forming products which volatilize.17. A polysilane which is substantially free of low molecular weightoligomers or cyclic materials and which comprises recurring units of

    --Si(X)(Y)--

wherein X and Y together have 5-13 carbon atoms, and X and Y eachindependently is hydrogen, alkyl, cycloalkyl, 1- or 2-naphthyl, orphenylalkyl, or together X and Y are an alkylene group forming a ringwith the adjoining Si atom, wherein X and Y groups which are not H canoptionally be substituted by one of C₁₋₆ -alkoxy, mono- or di-(C₁₋₃-alkyl)amino, C₂₋₆ -alkanoyl or the corresponding C₂₋₃ -ketal thereof,C₁₋₆ -alkyl, --NH₂ or --OH; and wherein, as a result of the selection ofX and Y, said polysilane has sufficient photosensitivity to depolymerizeupon exposure to actinic radiation, forming products which volatilize.18. A polymer of claim 1, wherein X and Y together have 6-13 C-atoms.